feat: ShellMate - SSH Chess TUI

Play chess in your terminal over SSH. No installs, no accounts.

Features:
- Beautiful terminal-filling chess board with ANSI colors
- Play against Stockfish AI (multiple difficulty levels)
- Two-step move interaction with visual feedback
- Leaderboard with PostgreSQL persistence
- SSH key persistence across restarts

Infrastructure:
- Docker containerized deployment
- CI/CD pipeline for dev/staging/production
- Health checks with auto-rollback
- Landing page at shellmate.sh

Tech: Python 3.12+, asyncssh, python-chess, Stockfish

CONTRIBUTING.md

@@ -0,0 +1,52 @@
+# Contributing to ShellMate
+
+Thanks for your interest in contributing! 🎉
+
+## Branch Strategy
+
+```
+feature/* ──→ develop ──→ master
+                │            │
+             staging      production
+```
+
+- **`develop`** — Default branch, all PRs target here
+- **`master`** — Production releases only
+- **`feature/*`** — Feature branches off develop
+
+## Development Flow
+
+1. Fork the repo
+2. Create a feature branch from `develop`:
+   ```bash
+   git checkout develop
+   git pull origin develop
+   git checkout -b feature/my-feature
+   ```
+3. Make your changes
+4. Run tests: `pytest tests/ -v`
+5. Run linting: `ruff check src/`
+6. Push and create a PR to `develop`
+
+## Code Style
+
+- Python 3.11+
+- Type hints required
+- Ruff for linting
+- 100 char line limit
+
+## Commit Messages
+
+Use conventional commits:
+- `feat:` New feature
+- `fix:` Bug fix
+- `docs:` Documentation
+- `refactor:` Code refactor
+- `test:` Tests
+- `chore:` Maintenance
+
+## Release Process
+
+1. PRs merged to `develop` deploy to staging
+2. When ready, merge `develop` → `master`
+3. Master deploys to production automatically